Hubble Spies Tiny Moons Circling Uranus

These images, taken with the NASA Hubble Space Telescope's Advanced Camera for Surveys (ACS), show several faint moons circling Uranus, including a newly detected moon and a rediscovered satellite. The planet's ring system can also be seen.

The arrow in the frame at right points to one of two newly discovered moons, among the smallest moons yet found around Uranus. The moon is temporarily designated as S/2003 U 1 until the International Astronomical Union (IAU) formally approves its discovery. S/2003 U 1 is orbiting 60,600 miles (97,700 km) away from the planet. If the satellite is as dark as Uranus's other moons, it is 10 miles (16 km) across, about the size of San Francisco. The Hubble telescope spotted this moon orbiting between the moons Puck, the largest satellite found by Voyager, and Miranda, the innermost of the five largest Uranian satellites. Astronomers previously thought this region was empty space. S/2003 U 1 whirls around the gas giant planet in 22 hours and 9 minutes.

The arrow in the frame at left points to a rediscovered moon orbiting 750 miles (1,200 km) away from the moon Belinda. The moon was detected in Voyager images, but the finding needed confirmation by an Earth-based telescope. Some astronomers think that S/1986 U 10 was once part of Belinda and broke off during a collision with a comet. Once certified by the IAU, these new discoveries will raise the number of Uranus moons to 24. Thirteen of them orbit even closer to Uranus than the five largest satellites, which are hundreds of miles wide. The location of one of those five satellites, Miranda, is shown in the image. The satellite itself cannot be seen because its bright light has been blocked out.

Astronomers stretched the limit of the ACS to find the S/2003 U 1 and S/1986 U 10. The moons are 40 million times fainter than Uranus. Even with the high resolution and sensitivity of the ACS, astronomers had to overexpose the images of Uranus to pinpoint the moons.

The images were made from a series of exposures taken Aug. 25, 2003. In order to show the faint moons in these images, the light from the much brighter Uranus has been blocked out. A separate but much shorter exposure of Uranus has been inserted into the images for reference. All the moons appear streaked because they were moving in their orbits during the long exposures. The white specks in the background are image artifacts.